Organic redox-active molecules characterized by electrochemical reversibility are used as electrode-active materials when they possess (1) chemical robustness for storage of a large amount of charge with cyclable charging/discharging properties, (2) sufficient mobility of charge-compensating electrolyte ions to allow the charge storage not only at the current-collector interface but also throughout the slab of the electrode-active materials, and (3) suitable swellability and insolubility in electrolyte solutions to maintain the anode|electrolyte|cathode configuration. Excellent nature of the materials in terms of cost and handling use is also essential. We have been pursuing a research to establish the organic electrode-active materials, employing various kinds of redox-active polymers. We have found that, based on the robustness in both oxidized and reduced (i.e. charged and discharged) states so-called as redox bistability, organic robust radicals, quinones, viologens, imides, and thianthrene derivatives are useful as the redox-active species. These molecules are used in cathodes and/or anodes, according to the redox potential and the electroneutralizing ions, giving rise to redox capacity comparable to the redox equivalent weight and full charging/discharging capability without fluctuating voltages, when electroneutralization by counterions is accomplished throughout the material.

The organic electrode-active materials are designed by incorporating the redox-active species as the pendant group of non-conjugated polymers per repeating unit. Polymethacrylates, polyacrylamides, polynorbornenes, polystyrene, and polyethers are typically employed for this purpose, imparting moldability and compatibility to wet fabrication process as well as the mass-transfer capability for charge compensation. Compact repeating units are especially advantageous to maximize the charge storage capacity.

Charge storage with the high density redox polymers proceed via electron self-exchange reaction of the redox-active sites. The exchange reaction-based transport process requires the facile electroneutralization. The driving force for the charge transport is a redox gradient which is governed by a diffusional process. Interestingly, the diffusive properties are dominated by a Brownian motion of the redox-active pendants in the polymer. In contrast to the electric conduction in doped Ɍ-conjugated polymers, a substantial current density is derived from the moderate carrier mobility and the very high carrier density.

The polymers are processed into a composite electrode containing a small amount of conducting additives such as nanocarbons and binders to fabricate the organic battery. Recently, we have established that the polymer nanoparticle dispersions act as flowing catholytes and anolytes to allow fabrication of an organic redox flow battery with high energy density. Another functionality of the redox polymers have also been found for their electrocatalytic activity as the mediator of inorganic electrode-active materials such as LiCoO₂ giving rise to fast charging properties. A flexible and stretchable organic batteries are fabricated with our polymers.